In nuclear reactors designed with passive operating systems, the laws of physics are employed to ensure that safe operation of the nuclear reactor is maintained during normal operation or even in an emergency condition without operator intervention or supervision, at 10 least for some predefined period of time. A nuclear reactor 5 includes a reactor core 6 surrounded by a reactor vessel 2. Water 10 in the reactor vessel 2 surrounds the reactor core 6. The reactor core 6 is further located in a shroud 122 which surround the reactor core 6 about its sides. When the water 10 is heated by the reactor core 6 as a result of fission events, the water 10 is directed from the shroud 122 and out of a riser 124. This results in further 15 water 10 being drawn into and heated by the reactor core 6 which draws yet more water 10 into the shroud 122. The water 10 that emerges from the riser 124 is cooled down and directed towards the annulus 123 and then returns to the bottom of the reactor vessel 2 through natural circulation. Pressurized steam 11 is produced in the reactor vessel 2 as the water 10 is heated.
A heat exchanger 35 circulates feedwater and steam in a secondary cooling system 30 in order to generate electricity with a turbine 32 and generator 34. The feedwater passes through the heat exchanger 35 and becomes super-heated steam. The secondary cooling system 30 includes a condenser 36 and feedwater pump 38. The steam and feedwater in the secondary cooling system 30 are isolated from the water 10 in the reactor vessel 2, such that they are not allowed to mix or come into direct contact with each other.
The reactor vessel 2 is surrounded by a containment vessel 4. The containment vessel 4 is designed so that water or steam from the reactor vessel 2 is not allowed to escape into the surrounding environment. A steam valve 8 is provided to vent steam 11 from the reactor vessel 2 into an upper half 14 of the containment vessel 4. A submerged blowdown valve 18 30 is provided to release the water 10 into suppression pool 12 containing sub-cooled water.
During a loss of feedwater flow, the nuclear reactor 5 is designed to respond by scramming the reactor core 6, flooding the containment vessel 4 or depressurizing the reactor vessel 2. The latter two of these responses result in the nuclear reactor 5 being shut down and unable to generate electricity for an extended period of time. Furthermore, during a loss of coolant condition where coolant is expelled from the reactor vessel 2, a flow of coolant through the reactor core 6 is reduced. This increases the time needed to bring the reactor core temperatures down to a desired level.